A hybrid vehicle powertrain generally employs multiple power sources, such as a conventional engine in combination with one or more electric motor/generators incorporated in a transmission, to generate output. A hybrid powertrain utilizing a transmission with two motor/generators may be arranged such that both torque and speed of the engine can be selected independently of vehicle speed and the desired acceleration. Thus, a two motor/generator hybrid powertrain may provide improved overall vehicle efficiency, as compared to a single motor/generator hybrid.
A hybrid that utilizes its transmission to provide additional, alternate ways or modes to power the vehicle via individual torque contributions from the engine and the motor/generator(s) is termed a multi-mode hybrid. In such an arrangement, selectable torque-transmitting clutches and brakes are typically employed to alter torque paths through the transmission's gear set(s), in order to vary mechanical advantage of the individual power sources relative to the output. Such an arrangement provides improved matching of torques and speeds of the power sources to existing speed and desired acceleration of the vehicle. For instance, it may be desirable to provide one mode for high-torque, low-speed operation, such as during launching a vehicle from rest, and one or more additional modes for high-speed, lower-torque operation.